The present invention relates to a process for the continuous preparation of polycarbonates and diaryl carbonates by the method of the phase boundary process in which both the mixing of the organic and aqueous phase and the upstream oligomerization step or the formation of the chloroformate of the monoaryl compound are effected in a special pump.
The polycarbonate preparation by the phase boundary process has already been described by Schnell, “Chemistry and Physics of Polycarbonates”, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, pages 33-70; D. C. Prevorsek, B. T. Debona and Y. Kesten, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960: “Synthesis of Poly(ester Carbonate) Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 18, (1980)”; pages 75-90, D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne', BAYER AG, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Volume 1 1, Second Edition, 1988, pages 651-692, and finally by Dres. U. Grigo, K. Kircher and P. R-Müller, “Polycarbonate [Polycarbonates]” in Becker/Braun, Kunststoff-Handbuch [Plastics Handbook], volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, Polyacetals, Polyesters, Cellulose Esters], Carl Hanser Verlag Munich, Vienna 1992, pages 118-145.
Furthermore, the phase boundary process for the preparation of polycarbonate is also described in EP-A 0 517 044 or EP-A 520 272:

For the preparation of polycarbonate by the phase boundary process, the phosgenation of a disodium salt of a bisphenol or a mixture of different bisphenols, initially introduced into aqueous alkaline solution or suspension, is effected in the presence of an inert organic solvent or solvent mixture which forms a second organic phase in addition to the aqueous phase. The resulting oligocarbonates mainly present in the organic phase are condensed with the aid of suitable catalysts to give high molecular weight polycarbonates dissolved in the organic phase, it being possible to control the molecular weight by suitable chain terminators (monofunctional phenols). The organic phase is finally separated off and the polycarbonate is isolated therefrom by various working-up steps.
The polycarbonate synthesis and the diaryl carbonate synthesis can be carried out continuously or batchwise. The reaction can therefore be effected in stirred tanks, tubular reactors, pumped circulation reactors or stirred tank cascades or combinations thereof, it being ensured by use of the abovementioned mixing members that aqueous and organic phase separate as far as possible only when the synthesis mixture has completely reacted, i.e. no longer contains any hydrolyzable chlorine from phosgene or chlorocarbonic acid esters.
The first step for the synthesis of the oligocarbonates is carried out according to the prior art, for example in a pumped circulation reactor; cf. for example EP 1 249 463 A1, US 2004/0158026 A1, U.S. Pat. No. 6,613,868 B2. In the pumped circulation reactor, the mixing of the introduced phosgene with likewise introduced disodium salt of a bisphenol (or of a mixture of different bisphenols) and the first oligomerization steps are effected. Resulting chloroformate groups react with terminal phenolate groups present to give growing oligomers which contain different terminal groups (phenolate or chloroformate or species mixed from the two). A certain proportion of bisphenol introduced is still present in unreacted form in the mixture. It is true that the pumped circulation reactor has advantages in that metering and concentration variations cannot continue unabated to the end product but rather an equilibrium of variations is achieved by back-mixing at a set circulation ratio for feed or discharge of the reaction mixture. However, there is a significant disadvantage in that relatively broad molecular weight distributions result for parameter settings which are advantageous on the industrial scale. A further disadvantage is that pumped circulation reactors are relatively bulky assemblies.
The preparation of diaryl carbonates (such as, for example, diphenyl carbonate) is usually effected by a continuous process, by preparation of phosgene and subsequent reaction of monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen catalyst at the boundary.

The preparation of diaryl carbonates, for example by the phase boundary process, is described in principle in the literature, cf. for example in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pages 50/51.
The laid-open application U.S. Pat. No. 4,016,190 describes a preparation process for diaryl carbonates which is operated at temperatures above 65° C. In this process, the pH is initially set low (pH=8 to 9) and then higher (pH=10 to 11).
Optimizations of the process by improvement of the mixing and maintenance of a narrow temperature and pH profile as well as isolation of the product are described in EP1219589 A1, EP1216981 A2, EP1216982 A2 and EP784048 A1.
For the preparation of diaryl carbonate by the phase boundary process, the phosgenation of an initially introduced sodium salt of a monophenol or of a mixture of different monophenols, in an aqueous alkaline solution or suspension, is effected in the presence of an inert organic solvent or solvent mixture which, in addition to the aqueous phase, forms a second organic phase. The resulting aryl chloroformates present mainly in the organic phase are converted with the aid of suitable catalysts to diaryl carbonates, dissolved in the organic phase. The organic phase is finally separated off and the diaryl carbonate is isolated therefrom by various work-up steps.
According to the prior art, the first step for the synthesis of the aryl chloroformates and/or diaryl carbonates is carried out, for example, in a pumped-circulation reactor; cf. for example EP 1 249 463 A1, US 2004/0158026 A1, U.S. Pat. No. 6,613,868 B2. In the pumped-circulation reactor, the mixing of the introduced phosgene with likewise introduced sodium salt of a monophenol (or of a mixture of different monophenols) to give aryl chloroformates and/or diaryl carbonates is effected. A certain proportion of the monophenol introduced is still present in unreacted form in the mixture. It is true that the pumped-circulation reactor has advantages in that metering or concentration variations cannot pass through undamped to the end product but rather an equilibration of variations is achieved by back-mixing at set pumped circulation ratio to feed or discharge of the reaction mixture. However, there is a significant disadvantage in that, for parameter settings advantageous in industry, there is still considerable incompletely reacted proportions of aryl chloroformates in the product composition. A further disadvantage is that pumped-circulation reactors are relatively bulky assemblies.
In these known processes, however, the high residual phenol value in the wastewater of these processes presents considerable disadvantages. Phenols can pollute the environment and cause increased wastewater problems for the wastewater treatment plants and necessitate complicated purification operations.
Thus, WO 03/070639 A1 describes removal of the organic impurities in the wastewater by extraction with methylene chloride.
Usually, the sodium chloride-containing solution is freed from solvents and organic residues and then has to be disposed of.
However, it is also known that, according to EP 1200359 B1 (WO2000078682 A1) or U.S. Pat. No. 6,340,736, the purification of the sodium chloride-containing wastewaters can be effected by ozonolysis and is then suitable for use in the sodium chloride electrolysis. A disadvantage of the ozonolysis is that this process is very expensive.
There was therefore a need for a continuous phase boundary process for the preparation of polycarbonate or diaryl carbonate, by means of which the pollution of the wastewater resulting from this process with diphenol or monophenol can be further reduced. Furthermore, it would be desirable if polycarbonates having narrower molecular weight distribution were obtained in such a process without it being possible, for example, for concentration variations to continue unabated to the end product (no plug flow behaviour). In the preparation of diaryl carbonates as well, this process prevents substantially unabated concentration variations of the aryl chloroformate up to the end product.
Surprisingly, it was found that a significant reduction in the diphenol or monophenol pollution of the wastewater can be achieved if both the mixing of the organic and aqueous phase and the upstream oligomerization step or the upstream chloroformic acid formation are effected in a special pump by the phase boundary process for the preparation of polycarbonates or diaryl carbonates. Advantageously, in this process too concentration variations do not continue unabated to the end product so that the process according to the invention retains advantages of the known process according to the prior art which eliminates disadvantages thereof. It is known that an increased diphenol and/or monophenol pollution of the wastewater requires, as a countermeasure, an increase in the phosgene and sodium hydroxide addition in order to complete the conversion of the diphenol or monophenol. Owing to the substantially reduced diphenol and/or monophenol pollution of the wastewater in the process according to the invention, it is therefore also possible to avoid such an increase in the phosgene and sodium hydroxide addition. This is advantageous, inter alia, from an economic point of view. Furthermore, the amount of aryl chloroformate decreases due to the more efficient reaction in the preparation of polycarbonates and diaryl carbonates, which requires a lower amount of amine catalyst and thus leads to a reduction in the urethane by-products.